A) Do this carefully. There are a lot of posts here from people who have lost an HDD password. And we are not allowed to answer them.
B) May I ask why? I can think of at least three ways, and they solve different issues.
1) Use HDPARM to set a drive password. This is a dangerous utility, and I can't tell you where you will be prompted for said password on boot.
2) Make the password match the BIOS password (I don't know how), and one password will unlock HDD and BIOS.
3) To protect some data, the disk or a portion thereof can be encrypted.